This invention relates generally to a teletype switching system for example, of the Telex type in which terminator units are provided for each customer or terminal that is connected to the system. More particularly, this invention concerns the generation of a stop bit by the terminator unit during the transmission of a character from a terminator unit to its terminal. The stop bit is generated at the end of each character transmitted and signifies to the terminal that the character has been sent.
Each terminal that is connected to the switching system communicates with its terminator unit either by direct wire connections or over a radio link. A typical Telex switching system uses a central processing unit (CPU) to control the communications between terminator units and consequently between terminals. The CPU is programmed to sequentially scan or pole each terminator unit, at a fixed sample rate, to inquire as to the status of the terminator unit and to receive or transmit data to the unit. In this way, if a terminator unit has received a character from its terminal and stored it in its memory, the CPU can, at the polling time for that terminator unit, transfer the character to another terminator unit. For the above conditions, the terminal which sent the character to its terminator unit is referred to as the calling terminal, while the terminal to which that character was sent is referred to as the called terminal. At the appropriate polling time, the character is transferred to the selected terminator unit by the CPU. The selected terminator unit then transmits the character to its terminal, thus completing the communication link between the calling and called terminals.
A character is typically transmitted between terminals at a 50 baud rate and consists of a five level code. This is commonly referred to as a baudot code. For a baudot code, each level of the character is transmitted serially beginning with a start bit and ending with a stop bit. The stop bit is characteristically equal to one and one-half bit-time intervals whereas the start bit and each of the five character level bits are one bit time. For a baudot code, the total transmission time for each character is 150 milliseconds. This corresponds to a character transmission rate of 400 characters per minute.
As long as the sending terminal is communicating with its terminator unit over wire lines, the prior-art methods of generating a stop bit for the character transmitted to a called terminal are acceptable. However, when the sending terminal is communicating with its terminator over a radio link, a problem arises in differences in transmission times between what is generated from the calling terminal to its terminator unit and that generated from the called terminator unit to its terminal. That is, the radio link increases the character transmission rate from calling terminal to terminator unit over that which results at the called terminal. Characters arrive at the calling terminator unit over the radio link at a rate of 411 characters per minute. The nature of a radio link requires a faster transmission rate because the radio link wants a little extra time to be available in case it has to request a re-transmission of a character out of the many that are being transmitted when transmission errors have been detected. Since the radio link buffers incoming characters from the calling terminal before transmission, the faster data-transmission time allows re-transmission of erroneous data without losing any of the characters continually being outputted by the calling terminal to the radio link buffer.
A faster transmission rate is accomplished by transmitting a stop bit length of 1-9/32 bit-time units instead of the normal 1-1/2 units. In a computerized Telex exchange the computer acts as a sampling device which transfers data from one terminator to another terminator. Therefore by basic communications theory the computer must be programmed to transfer characters at a rate equal to or greater than the character rate. By the same reasoning, the called terminator will output characters to its terminal at a rate equal to or faster than the character rate arriving at the calling terminator. Since the computer samples at a rate faster than the incoming character rate, there will be times when the computers samples the calling terminator and there will not be a character ready for transfer. Likewise, the called terminator, by outputting characters at a faster rate than it receives them, will catch up and have to wait. However, this faster character transmission rate causes the character printing at the receiving terminal to appear erratic because the called terminal periodically waits on the next character from its terminator unit. This results because the called terminator is outputing a shortened stop bit character to get a faster transmission and the CPU must skip a polling time before sending the next character to the called terminator unit.
The called terminal eventually catches up, but must wait until the next polling time before it can receive its next character. This waiting period is reflected as an objectionable erratic printing characteristic at the terminal. Also, operation with a continuously shortened stop bit increases the probability for garbling of the date in the telex system. Therefore, it would be advantageous to provide a terminator unit that could vary the length of the stop bit for each character transmitted between the called termintor unit and its terminal so that that transmission rate is essentially equal to the rate at which the calling terminator unit is receiving the characters over the radio link.
It would also be advantageous to provide a variable stop bit whose length is varied over a certain range in response to a pattern in the number of characters that are transmitted between skips in the character transmission (the group of characters between skips is referred to as a frame). In this manner, the stop bit length will be adjusted dynamically as the rate of transmission of characters from a calling to a called terminal varies due to the presence of a radio link.